Trigger on open database

if i put this script into a button on a table form it works.

do as server
    for a in select JobDockets where 'Job Bag' <= 13980 do
        a.(SupplierQuote := null);
        a.(SupplierConfirmation := null);
        a.(SupplierVisual := null);
        a.(Misc := null)
    end
end

However - Ideally, I want this to auto run when I open the database. However if i put it into options - trigger after open . The script is accepted and runs on open but it don't work and do update takes place

any suggestions

P.s I am using a set number for testing on job bag but will replace with something like 

 for a in select JobDockets where 'Job Bag' <= 'Job Bag' - 400 do (or along those lines)